Eager for a major legislative win, the Trump administration and Republican-controlled U.S. Congress are planning a massive tax cut, the biggest rewrite of the U.S. Tax Code since 1986.

Between now and the end of the year, in a legislative and political process pregnant with uncertainty, some taxpayers are likely to come out as winners and others as losers. Year-end tax planning, which is pretty much the same exercise every year, has been upended, and ignoring now how proposed changes to tax rules might affect your personal situation could result in a costly mistake this year.